Referring to FIG. 1, a conventional laser light source using an optical wavelength conversion element will be described. The laser light source is basically composed of a semiconductor laser 20, a solid state laser crystal 21 and an optical wavelength conversion element 25 made of KNbO.sub.3, which is a non-linear optical crystal.
As shown in FIG. 1, pumped light P1a emitted from the semiconductor laser 20, which oscillates at 807 nm, is collected by a lens 30 so as to excite YAG as a solid state laser crystal 21. A total reflection mirror 22 is formed on an incident surface of the solid state laser crystal 21. The total reflection mirror reflects 99% of light having a wavelength of 947 nm but transmits light in the 800 nm wavelength band. Although the pumped light P1a is thus efficiently introduced into the solid state laser crystal 21, the light with a wavelength of 947 nm, which is generated by the solid state laser crystal 21, is reflected to the optical wavelength conversion element 25 side without being emitted to the semiconductor laser 20 side. Moreover, a mirror 23, which reflects 99% of light having a wavelength of 947 nm but transmits light in the 400 nm wavelength band, is provided on the output side of the optical wavelength conversion element 25. These mirrors 22 and 23 form a resonator (cavity) for light having a wavelength of 947 nm, capable of generating oscillation at 947 nm as a fundamental wave P1.
The optical wavelength conversion element 25 is inserted in the cavity defined by the mirrors 22 and 23, whereby a harmonic wave P2 is generated. The power of the fundamental wave P1 within the cavity reaches to 1 W or higher. Therefore, the conversion from the fundamental wave P1 to the harmonic wave P2 is increased, whereby a harmonic wave having a high power can be obtained. A harmonic wave of 1 mW can be obtained by using a semiconductor laser having an output of 500 mW.
Next, referring to FIG. 2, a conventional optical wavelength conversion element having an optical waveguide will be described. The illustrated optical wavelength conversion element, when a fundamental wave having a wavelength of 840 nm is incident thereupon, generates a secondary harmonic wave (wavelength: 420 nm) corresponding to the fundamental wave. Such an optical wavelength conversion element is disclosed in K. Mizuuchi, K. Yamamoto and T. Taniuchi, Applied Physics Letters, Vol 58, p. 2732, June 1991.
As shown in FIG. 2, in this optical wavelength conversion element, an optical waveguide 2 is formed in an LiTaO.sub.3 substrate 1, with layers whose polarization is inverted (domain inverted layers) 3 being periodically arranged along the optical waveguide 2. Portions of the LiTaO.sub.3 substrate 1 where the domain inverted layer 3 is not formed will serve as a domain non-inverted layer 4.
When the fundamental wave P1 is incident upon one end (an incident surface 10) of the optical waveguide 2, the harmonic wave P2 is created in the optical wavelength conversion element and is output from the other end of the optical waveguide 2. At this point, light propagating through the optical waveguide 2 is influenced by a periodic structure formed by the domain inverted layers 3 and the domain non-inverted layer 4, whereby propagation constant mismatching between the generated harmonic wave P2 and the fundamental wave P1 is compensated by the periodic structure of the domain inverted layers 3 and the domain non-inverted layer 4. As a result, the optical wavelength conversion element is able to output the harmonic wave P2 with a high efficiency.
Such an optical wavelength conversion element includes, as a basic component, the optical waveguide 2 produced by a proton exchange method.
Hereinafter, referring to FIG. 3, a method for producing such an optical wavelength conversion element will be described.
First, at step S10 in FIG. 3, a domain inverted layer formation step is performed.
More particularly, a Ta film is first deposited so as to cover the principal surface of the LiTaO.sub.3 substrate 1, after which ordinary photolithography and dry etching techniques are used to pattern the Ta film into a striped pattern, thereby forming the Ta mask.
Next, a proton exchange process is performed at 260.degree. C. for 20 minutes for the LiTaO.sub.3 substrate 1 whose principal surface is covered by the Ta mask. Thus, 0.5 .mu.m thick proton exchange layers are formed in portions of the LiTaO.sub.3 substrate 1 which are not covered by the Ta mask. Then, the Ta mask is removed by etching for 2 minutes using a mixture containing HF:HNF.sub.3 at 1:1.
Next, a domain inverted layer is formed within each of the proton exchange layers by performing a heat treatment at 550.degree. C. for 1 minute. In the heat treatment, the temperature rise rate is 50.degree. C./sec and the cooling rate is 10.degree. C./sec. In portions of the LiTaO.sub.3 substrate 1 where the proton exchange has been performed, the amount of Li is reduced as compared to that in other portions thereof where the proton exchange has not been performed. Therefore, the Curie temperature of the proton exchange layer decreases, whereby the domain inverted layer can be formed partially in the proton exchange layer at a temperature of 550.degree. C. This heat treatment allows for formation of the proton exchange layer having a pattern upon which the pattern of the Ta mask is reflected.
Next, at step 2 in FIG. 3, an optical waveguide formation step is performed.
More particularly, step 2 is generally divided into step S21, step S22 and step S23. The mask pattern is formed at step S21; the proton exchange process is performed at step S22; and high-temperature annealing is performed at step S23.
These steps will be described below.
At step S21, the Ta mask used for forming the optical waveguide is formed. The Ta mask is obtained by forming slit-shaped openings (width: 4 .mu.m, length: 12 mm) in a Ta film. At step S22, a high refractive index layer (thickness: 0.5 .mu.m) linearly extending in one direction is formed in the LiTaO.sub.3 substrate 1 by performing a proton exchange process at 260.degree. C. for 16 minutes for the LiTaO.sub.3 substrate 1 which is covered by the Ta mask. The high refractive index layer will eventually function as an optical waveguide. However, the non-linearity of the portions where the proton exchange has been performed (the high refractive index layers), as thus formed, is deteriorated. In order to restore the non-linearity, annealing is performed at 420.degree. C. for 1 minute at step S22 after removing the Ta mask. This annealing expands the high refractive index layer in the vertical direction and in the lateral direction, thereby diffusing Li into the high refractive index layers. By reducing the proton exchange concentration in the high refractive index layers in this way, it is possible to restore the non-linearity. As a result, the refractive index of the regions located directly under the slits of the Ta mask (the high refractive index layers) is increased by about 0.03 from the refractive index in other regions, whereby the high refractive index layers function as an optical waveguide.
Next, a protective film formation step (step S30), an end face polishing step (step S40), and an AR coating step (step S50) are performed, thereby completing an optical wavelength conversion element.
By setting the arrangement pitch of the domain inverted layers periodically arranged along the waveguide to 10.8 .mu.m, it is possible to form a third-order pseudo phase-matched structure.
With the above-described optical wavelength conversion element, when the length of the optical waveguide 2 is set to 9 mm, the harmonic wave P2 having a power of 0.13 mW can be obtained for the fundamental wave P1 (power: 27 mW) having a wavelength of 840 nm (conversion efficiency: 0.5%).
For forming a first-order pseudo phase-matched structure, the arrangement pitch of the domain inverted layers can be set to 3.6 .mu.m. In this case, the harmonic wave P2 of 0.3 mW can be obtained for the fundamental wave P1 of 27 mW (conversion efficiency: 1%). The inventors of the present invention have experimentally produced a laser light source which outputs blue laser light by combining such an optical wavelength conversion element with a semiconductor laser.
Such an optical wavelength conversion element has a problem that the phase-matched wavelength thereof varies with the passage of time, whereby a harmonic wave cannot be obtained. When the wavelength of the fundamental wave emitted from a semiconductor laser is kept constant, but the phase-matched wavelength of the optical wavelength conversion element is shifted, the harmonic wave output will gradually decrease, and it will eventually becomes zero.
The object of the present invention is to stabilize a laser light source, to increase the output thereof, and to reduce the size and weight of a laser device or an optical disk apparatus by incorporating a high output laser light source into these devices/apparatuses.